Lithium secondary batteries are excellent in energy density, output density, etc. and are effective in size and weight reduction. There is hence a rapidly growing demand for these batteries as power sources for portable appliances such as notebook type personal computers, portable telephones, and handy video cameras. Lithium secondary batteries are receiving attention also as power sources for electric cars, power load leveling, etc.
At present, lithium-manganese composite oxides having a spinel structure, layered lithium-nickel composite oxides, and layered lithium-cobalt composite oxides are used as positive-electrode active materials for lithium secondary battery. Lithium secondary batteries employing these lithium composite oxides each has advantages and disadvantages in properties. Namely, the lithium-manganese composite oxides having a spinel structure are inexpensive and relatively easy to synthesize and give batteries having excellent safety, but these batteries have a low capacity, and poor high-temperature characteristics (cycle and storage). The layered lithium-nickel composite oxides have a high capacity, and excellent high-temperature characteristics, but have disadvantages, for example, that they are difficult to synthesize and give batteries which have poor stability and necessitate care in storage. The layered lithium-cobalt composite oxides are easy to synthesize, excellent in battery performance balance, and hence extensively used as power sources for portable appliances but have great disadvantages in that the cells have insufficient safety and a high cost.
Under these circumstances, a lithium-nickel-manganese-cobalt composite oxide having a layered structure was proposed as a promising active material which eliminates or minimizes the disadvantages of those positive-electrode active materials and attains an excellent battery performance balance. In particular, there recently are growing desires for a cost reduction and higher safety, and this composite oxide is thought to be a promising positive-electrode active material capable of satisfying these two desires. It should, however, be noted that the degrees of cost reduction and safety vary depending on composition, in particular Ni/Mn/Co proportion, and it is therefore necessary that a material having a composition in a limited range specified by the present inventors should be selected and used in order to satisfy the desires for a further cost reduction and higher safety.
However, lithium secondary batteries in which a layered lithium-nickel-manganese-cobalt composite oxide having a composition in such a range for a low cost and high safety are reduced in battery performances themselves such as charge/discharge capacity and output characteristics. A further improvement has hence been necessary for improving battery performances for practical use.
Among patent documents in which a battery performance improvement was attempted in a lithium-nickel-manganese-cobalt composite oxide composition region with relatively high safety is JP-A-2002-110. One of the reasons for a battery performance improvement in this patent document may be the selection of a lithium/transition metal (nickel, manganese, and cobalt) proportion. However, there is no statement in this document on volume resistivity, which is a requirement for battery performance improvement in the invention. Furthermore, there is no statement concerning the concentration of the carbon contained, which as an impurity component causes side reactions and is present on the surface of and at grain boundaries in the positive-electrode active material to inhibit lithium ion intercalation/deintercalation reactions and which thereby or otherwise influences the battery performances. The document includes no recognition to influences of volume resistivity and the concentration of carbon contained on battery performances.
In addition, the process for production through coprecipitation described in that document has a drawback that the coprecipitated hydroxide precursor obtained has a small specific surface area and hence shows low reactivity with a lithium compound in the calcination step. Unreacted lithium is hence apt to remain in the form of carbonate. Furthermore, since subsequent mixing with the lithium compound is insufficient, the excess lithium ingredient will remain as a carbonate in parts where the lithium compound localizes. It is therefore extremely difficult to obtain a lithium-nickel-manganese-cobalt composite oxide having the low carbon concentration specified in the invention.
Patent Document 1: JP-A-2002-110167